To date, the semiconductor industry has been able to produce IC wafers increasing functional capabilities and increasing density without necessarily suffering losses during the transport processes, or at least having not realized the packaging media as a source for those losses. In general, present day transport media designed for packaging IC wafers are lacking the necessary features to address several problems common to advanced technology wafers during insertion and transport. This is especially true for wafers having higher speed with smaller geometries and having elevated interconnect members including bond pads, caps, and balls. These problems can manifest themselves in the form of disfigured connectors that include wafer breakage, scratch damage as well as mobile ion induced parametric failures.
Wafer shipping containers/boxes in combination with bags, outer cardboard type boxes, cushions and separators that are not functionally coordinated nor objectively systematized to address wafer movement, Airborne Molecular Contaminants (AMCs), and vapor leakage during transport can cause yield problems to the semiconductor wafers. Yield problems are associated with the following problems.
Wafer Movement:
Wafer boxes/containers currently utilize oversized vertical wall configuration to accommodate insertion of wafers without restrictions. The walls normally are unable to move inward to take up the slack after the box is fully loaded. The resulting wafer movement, when combined with wafers that utilize soft thin protective overcoats and elevated soft pads, caps and/or ground rings, can result in scratching during the wafer insertion and transport process. Stacked wafers with elevated features may also transfer structural damage to other associated wafers if improper materials are selected with too soft or too stiff a compressibility factor. Partial loading of a box changes the compressibility requirements of the system so that simply adding more cushions may not be the most appropriate solution.
The movement of wafers within the transport media generate shaved particles that enhance scratch damage and promote slough particles.
These particles, with the presence of AMCs, can further enhance the possibility for surface contamination. These contaminates may lead to corrosive damage and/or transistor inversion.
AMCs are exceptionally small in size they are generally corrosive and they carry a charge. Through molecular migration, a charge build may occur over an active transistor node resulting in transistor inversion and a parametric failure. These type defects generally are latent in nature, may be identified at final test, but usually appear as field returns or through extended life test analysis. The failure mechanism will disappear upon removal of the encapsulation media, removing any evidence that may suggest the source of the contamination. There will be no clear path leading back to the transport media system as a source of the problem.
Scratch Damage During Insertion
A robotic system transferring a wafer through the insertion process allows some lateral movement of the inserted wafer. This movement (from direct drop or placement) transfers through the underlying separator and to the top surface of the covered wafer. The impact, depending on the weight of the wafer and the amount of trapped air, will result in some amount of uneven force as the two surfaces come in contact with each other. The allowed lateral movement during the insertion will result in scratch damage. These scratches are typically sub-micron in size and may further migrate through the passivation oxide when cushion compressive forces are developed while closing the box. This type of crazing damage is not necessarily catastrophic, and it is unknown if such forces act to create catastrophic failure during extended life testing. Nor is it understood if this sub-micron crazing can later become a point of entry for corrosive growth. It is known that such damage has been witnessed at the bevel edges of the wafers.
Scratch Damage: Smeared or Scratched Circuit Lead Scratches
During transport, lateral movement of wafers within containers/boxes will scratch wafer surfaces during shipment. The resulting scratches will cause damage to interconnect circuitry including smashing and disfiguring elevated connecting members such as ground rings, ball bond pads, and caps. These scratches can form shorts from one metalized area to another. The same lateral movement will also create shaving from the protective separators which further promotes scratch damage.
Wafers packaged within boxes should have no allowance of lateral motion during shipment phases to avoid concerns of damaged elevated circuitry.
Corrosive Damage
Wafers packaged within enclosures such as boxes and bags that have a high level of MVTRs (moisture vapor transport rate) when trapped within those enclosures that excessively out-gas Airborne Molecular Contaminants (AMCs), in combination can create corrosive residues that are able to settle on the surface. Those corrosive residues can move in the direction of bond pads and any other exposed metallization to create damage, either immediately or as a latent defect when later placed used bias and able to migrate over an active node to create transistor inversion.
The amount of corrosive damage that transfers to a surface depends upon the abundance of AMCs that associate with the packaging materials and barometric pressure, temperature and relative humidity that modifies the MVTR assigned to the boxes and bags containing the wafers. Evidence of corrosion entry include (1) edge & bevel missing metal, (2) lifted pads, (3) stained pads, and (4) dark corroded pads.
Within a finished 16/300 dip product, the molecular transfer of hydrogen and oxygen (H2O) molecules through the encapsulate occurs under 168 hours at room ambient and 50% RH. The finished product, when placed under bias, activates molecular movements of the ions which tend to migrate to various transistor nodes. Assuming no cracks or crazing have occurred to the passivation, the charge build that gathers above the transistor node may result in the transistor inverting, leading to a parametric circuit failure. The rate of mobilization depends on the bias voltage, time of on state, and content of AMCs within the vapor transfer at the passivation surface. Processes leading up to the encapsulation process do not normally impact the attachment of these charged ions on the surface of the passivation. Generally these AMCs have already attached themselves to the oxide so that saw and grind slurry and their respective cleanups accomplish little to achieve removal.
Stained Bond Pads
For wafers packaged within shipping containers, there are instances where bond pads and adjacent passivation coatings will accumulate contamination that appears as a stain. The stain appears to extend beyond bond pads under the passivation coated areas. This contamination condition seems to be traceable to a mismatch between photo-resist and the passivation coating usually found in the bond pad areas. Due to the mismatch of the passivation, a chemical reaction driven by the presence of moisture vapors combined with organic type AMCs, such as contaminating hydrocarbons, allows for the first stage of corrosion to begin.
Clean rooms are teeming with AMCs that cannot be effectively removed by HEPA filters. When wafers are packaged within boxes having moisture vapors that have not been fully removed, those bags become carriers for AMCs, settling on all surfaces including bond pads and over-coating passivation. A small amount of chemical reaction takes place with the exposed aluminum or copper surface, thereby resulting in a corrosive stain in the area of bond pads as well as in any area where a mismatch between the photo-resist and the PO coating occurs.
Corrosive Bond Pads
Surfaces of bond pads that become excessively corroded while in transit from one location to another may become unnecessarily exposed to the condition of AMCs. This damage is normally restricted to bond pad surfaces only and normally is associated with the presence of moisture vapors. Sources with the transport system may include the out-gassing of cushions and/or separators. This out-gassing may be linked to inorganic and organic type AMCs resulting in the corrosive damage.